1. Field of the Invention
The present invention relates to color sensors, and more particularly, to sensors for measuring the reflective and fluorescent properties of various objects such as paper.
2. Description of the Prior Art
In industry, it is often important to accurately measure the color of an object as it is being manufactured. For instance, purchasers of paper frequently require the paper color to accurately match the color of previous purchases. Thus, paper manufacturers need to measure and control the color of paper to a previously determined value.
In the absence of fluorescence, the whiteness or color of an object is in general determined by the way the object absorbs and reflects light across the visible spectrum which when defined in terms of wavelength, is approximately 380 to 780 nanometers (nm). For example, white objects reflect light evenly across the spectrum while colored objects absorb some wavelengths (or color) and reflect others.
Color sensors typically illuminate the object and measure the intensity of the reflected light at each of a number of wavelengths. Each measured intensity of reflected light can be related to a previously measured intensity of light which has been reflected from a white standard to provide a reflectance co-efficient at each wavelength. The set of reflectance co-efficients is often referred to as the color spectrum of the object.
Because paper naturally has a somewhat offwhite or straw color, fluorescent whitening agents (FWA) in the form of dyes are often added to the paper pulp to give the finished paper product a whiter appearance. Fluorescence is the phenomenon in which energy is absorbed over a range of wavelengths (the excitation band) and then reemitted in a lower energy (longer wavelength) band referred to as the emission band. Fluorescent whitening agents typically absorb the violet and ultraviolet energies and remit this energy in the blue range to give the paper a whiter appearance.
The degree to which the object or paper illuminated by a particular source is made to appear bluer (or whiter) by the FWA depends upon the proportion of the energy emitted by the source in the violet and ultraviolet regions as compared to the blue and other lower energy spectra, and also upon the effective concentration level of the fluorescent agent. Thus, if the excitation emission spectrum of the source changes, the amount of fluorescence and hence the color of the object will change. The emission spectra of various standard sources have been defined by the Commission Internationale de L'Eclairage (CIE) but attempts to build standard sources which exactly duplicate the defined spectra have been largely unsuccessful. As a result, the emission spectra of the sources used in color sensors usually varies from sensor to sensor so that measured color spectra for a given fluorescent object also varies, although they may agree quite satisfactorily on non-fluorescent objects.
The effective FWA concentration level in an object has been measured using highly sophisticated techniques. In one such technique, a standard object having a known FWA concentration is first illuminated by a source and the intensity of the light received from the object is measured. The object is then reilluminated after a filter which eliminates the light in the excitation band is placed over the source, and a second measurement is taken. The difference in these two measurements is directly related to the effective FWA concentration in the standard. The process is repeated for a sample of unknown FWA concentration and the two difference measurements are compared to determine the effective FWA concentration in the sample.
This technique is typically very cumbersome since it involves the mechnical movement of a filter in front of the source and is therefore relatively slow. Consequently, it is difficult to rapidly measure the FWA concentration which makes the technique less practical for many on line applications such as measuring the FWA concentration in paper as it emerges from the paper making process. Also since the paper has moved substantially between the two intervals of measurement, it cannot be assumed that the difference is solely a function of the FWA concentration. Any variation in a number of other attributes of the paper, such as opacity or whiteness, at the two points of measurement, could also affect the results.